Replace deprecated 'error-chain' with 'thiserror' (#1820)

We can't use #[from] on Error::Msg(String) because String does not implement Error.
(Which it shouldn't; see e.g. https://internals.rust-lang.org/t/impl-error-for-string/8881.)
So we implement From manually for Error::Msg, since our current code was written
in that way for error-chain.

CHANGELOG.md

@@ -31,6 +31,7 @@
 
 - Deprecate `HighlightingAssets::syntaxes()` and `HighlightingAssets::syntax_for_file_name()`. Use `HighlightingAssets::get_syntaxes()` and `HighlightingAssets::get_syntax_for_file_name()` instead. They return a `Result` which is needed for upcoming lazy-loading work to improve startup performance. They also return what `SyntaxSet` the returned `SyntaxReference` belongs to. See #1747, #1755 and #1776 (@Enselic)
 - Remove `HighlightingAssets::from_files` and `HighlightingAssets::save_to_cache`. Instead of calling the former and then the latter you now make a single call to `bat::assets::build`. See #1802 (@Enselic)
+- Replace  the `error::Error(error::ErrorKind, _)` struct and enum with an `error::Error` enum. `Error(ErrorKind::UnknownSyntax, _)` becomes `Error::UnknownSyntax`, etc. Also remove the `error::ResultExt` trait. These changes stem from replacing `error-chain` with `thiserror`. See #1820 (@Enselic)